1. Technical Field of the Invention
The present invention generally relates to telecommunications systems, and in particular, to a telecommunication system that includes a subsystem for providing call services in the event of a communication failure.
2. Description of the Related Art
Conventional telecommunication systems typically includes numerous signal transfer point devices, such as switches, line trunk units (LTUs), and remote communications modules (RCMs), coupled together by a multitude of telecommunication line links. FIG. 1 illustrates some of the components commonly found in prior art telecommunication networks. The network 10 includes a main switch 12 connected to a remote shelf 14 using a conventional communications link 16, such as an isochronous circuit-switched network. Typically, the main switch 12 can be connected to additional remote shelves, as well as to the central office (CO) of a common carrier using a public switched telephone network (PSTN). One or more peripheral units (not shown), such as telephones, modems, faxes, etc., can be connected to the remote shelf 14.
The main switch 12 can be a networked switch for providing switching and call setup services to the remote shelf 14. For example, the main switch 12 can be implemented using a computerized branch exchange (CBX) or private branch exchange (PBX), such as the Hicom 300E manufactured by Siemens Corporation. A PBX typically provides bandwidth and switching capacity to one or more remote shelves and thus, a PBX can act as a central switching hub for several remote shelves.
The remote shelf 14 includes a plurality of analog or digital ports and interface circuits for connecting a group of peripheral units. Typically, the remote shelf 14 has a relatively limited functionality, and it relies on the main switch 12 for most call services, such as call set-up and switching. Consequently, all calls placed between peripheral units and the CO are routed through the main switch 12. The remote shelf 14 can be an LTU or RCM, such as one manufactured by Siemens Corporation, located several kilometers (up to 700 miles) from the main switch 12.
The communications link 16 interconnects the remote shelf 14 and the main switch 12 using either a fiber-optic or copper cable. Typically, the main switch 12 and remote shelf 14 are interconnected using a circuit-switched network protocol, such as the T1, T2, T3, and T4 carriers developed by the Bell System, or the E1, E2, E3 carriers established by CCITT. These standard protocols are isochronous, that is, data is transferred between devices at the same average bit rate.
In addition to being isochronous, conventional telecommunications networks are designed to handle pulse-coded modulated (PCM) voice as well as signalling information. PCM voice is a digital representation of voice input or computer data that is transmitted through a conventional analog telephone input using a modem. Signalling information is digital data that does not represent PCM voice and is generally used for call set-up. Call setup is the process of reserving network resources, such as channel time slots and lines, for establishing a point-to-point circuit to effectuate the call. The signalling information can be transmitted in-band or out-of-band. In-band signalling uses the same communication channel as PCM voice/data to accomplish call setup, while out-of-band signalling uses a separate channel, such as a common channel, to set up calls.
Telecommunications equipment generally represents a large capital expense and has a long service life. Accordingly, many installed telecommunication networks include older equipment which lacks fault recovery features for enhancing service reliability. In these systems, a communication failure between the main switch 12 and remote shelf 14 can cause users connected to the remote shelf 14 to lose all call services. The communication failure can be caused by failure of either the main switch 12 or the communication link 16.
In such a system, to maintain call services at the remote shelf 14 in the event of a failure, it is necessary to modify either the remote shelf 14 or the main switch 12 to include backup services that permit a degree of fault tolerance. However, modifying the functionality of the main switch 12 and remote shelf 14 typically requires replacing these devices, as most older telecommunication equipment is not designed to be incrementally upgraded. In many instances, such a costly upgrade is unacceptable. An alternative solution would be to provide a networked switch, such as a PBX, at the remote shelf 14 location. In this arrangement, remote users would no longer rely on the main switch 12 for call services, and a failure of the main switch 12 or link 16 would not lead to a cessation of remote services. However, this solution is not a cost-effective alternative for remote locations having a small number of users.
Therefore, there is a need for an apparatus that can be easily integrated into an existing telecommunications network to provide backup call services to remote users in the event of a communication failure, without requiring expensive upgrades to existing telecommunication equipment.
It is an advantage of the present invention to provide an apparatus that maintains call services at a remote location in the event of a communication failure with a main switch. It is also an advantage of the present invention to provide an apparatus that can be easily integrated into an existing network without costly upgrades to the previously installed telecommunication equipment.
The present invention provides a fail-safe subsystem that can be connected to a communication link between a main switch and a remote shelf. During normal operation, the subsystem remains in a pass-through mode. In pass-through mode, the subsystem passively monitors and stores all call information passed between the main switch and the remote shelf. In the event of a communication failure, the subsystem activates to maintain virtually uninterrupted call services to the remote shelf. To accomplish this, the subsystem includes a local digital switching matrix, which provides local switching services to remote users based on the stored call setup information. Upon restoration of communications between the main switch and remote shelf, the fail-safe subsystem re-configures the main switch with the current calling status of the remote shelf before returning to its pass-through mode of operation. Thus, the main switch and remote shelf can be re-connected in a relatively seamless manner without affecting calls established during the failure.